A Group of Astrophysicists Casts Doubt on the Kerr Hypothesis, Proving That Black Holes Continue to Deliver Surprises

  • The gravitational collapse of a massive star isn’t the only mechanism that can explain the formation of a black hole.

  • Mathematician Roy Kerr’s research has significantly improved the scientific community’s understanding of black hole dynamics and the principles of gravitational relativity.

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Black holes are among the most enigmatic objects in the universe, rivaling only neutron and quark stars in prominence. The scientific community’s understanding of them is still limited, but cosmologists are gradually uncovering some of their mysteries.

A black hole is a finite region of space that contains enough mass to generate a gravitational field so intense that not even photons (light particles) can escape from it. While several types of black holes exist, astrophysicists study cosmic black holes most extensively. These black holes form when very massive stars collapse.

The mass of a star determines its fate at the end of its life. Less massive stars typically give rise to nebulae, leaving behind a white dwarf at their center. A white dwarf is a degenerate star that has exhausted all its fuel and is much smaller than its initial size. In contrast, the most massive stars may end their lives as neutron stars, quark stars, and, if they’re sufficiently massive, black holes.

The Kerr Hypothesis Has a Problem

In 1963, New Zealand mathematician and physicist Roy Kerr significantly contributed to theoretical physics by finding a solution to Albert Einstein’s field equations that accurately describes a rotating black hole. Prior to this, the only framework available to understand black holes was the Schwarzschild metric by German physicist Karl Schwarzschild, which represents a spherical black hole that can’t rotate. Astrophysicists soon recognized that Kerr’s model was more credible.

Astrophysicists have observed that most cosmic objects possess rotational motion due to the angular momentum they retain from their formation.

For several reasons, the Kerr metric is more general and realistic than the Schwarzschild solution. One key aspect is that it describes a black hole with an event horizon, a region of space surrounding it beyond which any object that crosses it will fall irretrievably into the black hole. Moreover, astrophysicists have observed that most cosmic objects possess rotational motion due to the angular momentum they retain from their formation.

Since stars, planets, neutron stars, and galaxies all rotate, it seemed reasonable to conclude that black holes should also have rotation. This alignment of theories made Kerr’s work essential in the field of astrophysics. Thanks to his contributions, researchers have gained a better understanding of the dynamics of black holes and gravitational relativity, which specifically studies how gravity affects space, time, and the motion of objects.

A recent scientific paper argues that the inner horizon of a black hole can’t store an infinite amount of energy.

Additionally, Kerr’s solution indicates that the event horizon isn’t the only boundary of a black hole. Inside is another region known as the Cauchy horizon, where the behavior of the space-time continuum becomes completely unpredictable due to the presence of a singularity. In the Cauchy horizon, the gravitational curvature becomes infinite, and the laws of physics known to date can’t be applied.

Recently, a group of astrophysicists from Italy, the Czech Republic, Denmark, and New Zealand published findings contradicting the Kerr hypothesis. In their paper, published in Physical Review Letters (and available on arXiv), they argue that the inner horizon of a black hole can’t store an infinite amount of energy. They claim that, during a black hole’s evolution, the accumulated energy will eventually reach a limit that destabilizes it.

This is the key point where the new theory proposed by the international group of astrophysicists diverges from the Kerr metric. The implication is significant: Black holes that consume matter during their active phases can’t persist indefinitely. Eventually, the energy accumulated at their inner horizon will lead to a destabilization.

However, this new theory also conflicts with observational data, given that astronomers have identified matter-gobbling black holes, or accretion disks, that are billions of years old. One notable example is the black hole at the center of the galaxy M87, which is approximately 13 billion years old.

Image | NASA

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